The present invention relates to an image forming apparatus which produces image data by reading an original document, modulates a light beam issuing from a light emitting element by the image data, and exposes an electrophotographic photoconductive element by the light beam modulated to thereby form an image. More particularly, the present invention is concerned with an image forming apparatus capable of rendering an image with multiple tones without degrading resolution.
Today, a digital copier which reads a document by a CCD (charge coupled device) line sensor or like image reader and applies the resulting data to, for example, a laser printer is extensively 8used. Documents applicable to such a digital copier include photographs and drawings each having multiple tones. In the case that image data produced by reading a document having tones are fed to a laser printer, it is a common practice to render the tones by using a dither matrix method which per se is well known in the art. The dither matrix method is such that one pixel is represented by a 4.times.4 matrix so as to render sixteen different tones. A greater number of tones are achievable by increasing the matrix size, e.g., sixty-four tones may be rendered by an 8.times.8 matrix.
A drawback with the prior art multi-tone representation scheme is that the increase in matrix size as needed to accommodate a greater number of tones results in coarse pixels and, therefore, poor resolution.
For details of this kind of image forming apparatus, a reference may be made to Japanese Laid-Open Patent Publication (Kokai) Nos. 61-189754/1986, 61-189575/1986, 61-189577/1986 and others.